The protein kinase C inhibitor sotrastaurin allows regulatory T cell function

Summary The novel immunosuppressant sotrastaurin is a selective inhibitor of protein kinase C isoforms that are critical in signalling pathways downstream of the T cell receptor. Sotrastaurin inhibits nuclear factor ( NF) ‐κB, which directly promotes the transcription of forkhead box protein 3 ( F ox P3) , the key regulator for the development and function of regulatory T cells (Tregs). Our center participated in a randomized trial comparing sotrastaurin ( n  = 14) and the calcineurin inhibitor N eoral ( n  = 7) in renal transplant recipients. We conducted ex vivo mixed lymphocyte reaction (MLR) and flow cytometry studies on these patient samples, as well as in vitro studies on samples of blood bank volunteers ( n  = 38). Treg numbers remained stable after transplantation and correlated with higher trough levels of sotrastaurin ( r  = 0·68, P  = 0·03). A dose‐dependent effect of sotrastaurin on alloresponsiveness was observed: the half maximal inhibitory concentration ( IC 50 ) to inhibit alloactivated T cell proliferation was 45 ng/ml (90 nM). In contrast, T reg function was not affected by sotrastaurin: in the presence of in vitro ‐added sotrastaurin (50 ng/ml) Tregs suppressed the proliferation of alloactivated T effector cells at a 1:5 ratio by 35 versus 47% in the absence of the drug ( P  = 0·33). Signal transducer and activator of transcription 5 ( STAT)‐ 5 phosphorylation in Tregs remained intact after incubation with sotrastaurin. This potent Treg function was also found in cells of patients treated with sotrastaurin: Tregs inhibited the anti‐donor response in MLR by 67% at month 6, which was comparable to pretransplantation (82%). Sotrastaurin is a potent inhibitor of alloreactivity in vitro , while it did not affect T reg function in patients after kidney transplantation.